Hundreds of millions of years ago, a common ancestor
of mammals, birds, and reptiles evolved a phallus.
We don't know much about phallus evolution (external
genitalia generally don't mineralize, so the fossil
record is of little help), but we can compare the
expression of phallus genes from organism to
organism. From such work, we've learned that many of
the genes deployed in the developing phallus are
also used to build limbs during embryonic
development.
Anolis lizard embryo expression of the hindlimb/phallus
gene Tbx4 (purple stain) - Credit Carlos R. Infante
The shared patterns of gene expression in the limbs
and phallus are generated in part by a common set of
noncoding DNA, also called "elements" or
"enhancers," which act to control gene expression in
both of these structures, argues a study published
October 1 in Developmental Cell. These conclusions
stemmed from an initial observation that many limb
control elements, or limb enhancers, found in limbed
animals are still present in snake genomes.
"From purely looking at the DNA sequences we can
conclude that snakes retain many enhancers that,
based on mammalian studies, we thought were limb
enhancers," says senior study author Douglas Menke,
a geneticist at the University of Georgia. "There
have been tens of millions of generations for these
elements to be lost, but the fact that these are
still present in snakes prompted us to rethink what
these limb enhancers are doing in snakes and mice."
Menke and postdoctoral researcher Carlos Infante
examined patterns of enhancer activity in embryonic
limbs and genitalia of mice and limbed reptiles
(anole lizards). This revealed that many of the same
enhancers are activated during the formation of
these different appendages in both species.
When the authors tested the ability of the lizard
and snake versions of this limb-genital enhancer to
function in mice, they found that the lizard version
was capable of driving gene expression in the legs
and the genitalia (much like the mouse version),
while the snake version was only capable of driving
gene expression in the genitalia.
The conclusion is that many of these noncoding
regions of DNA should be more broadly categorized as
"appendage enhancers" rather than "limb enhancers,"
and snakes may have retained these noncoding DNA
elements due to their role in phallus development.
Menke had access to the genomes of three snake
species for his study: boa constrictor, Burmese
python, and king cobra. Comparative genomics
research like this has only been possible in the
past couple of years as the genome sequences of
snakes and other species have become available. One
of the next steps will be to investigate how much of
a role noncoding DNA plays in the formation of
different genital shapes that are observed in
nature, from the dual hemiphalluses found in lizards
and snakes to the diversity of morphologies observed
among the phalluses of mammalian species.
For more information
Developmental Cell, Infante et al.: "Shared Enhancer
Activity in the Limbs and Phallus and Functional
Divergence of a Limb-Genital cis-Regulatory Element
in Snakes"
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